With wind speeds of up to 140kmph, Yaas had the makings of an extreme cyclone as it made landfall on India’s eastern coast. Close to 15 lakh people have been moved to safety across the two states of Odisha and Bengal, which lie in the path of the cyclone that is the second to strike India in as many weeks after Tauktae lashed the western coast in the middle of May.
Pressure tactics: How cyclones form?
Late April and May represent a time of peak cyclonic activity in the Indian Ocean, months which coincide with the hot Indian summer. And heat has a lot to do with how these tropical cyclones arise.
Tropical cyclones form only over warm ocean waters near the equator. The way it works is this: warm, moist air over the ocean rises upwards from the surface, leading to a vacuum-like situation that causes a low air pressure area to form below.
When that happens, cooler air from the surrounding area rushes in to replace the warm air that is rising. But this new, cooler air, too, becomes heated and rises. This cycle of hot air rising up and being replaced by the cooler air leads to the formation of a cloud system over the area, which spins and grows as it is fed by the ocean’s heat and water evaporating from its surface.
This system now has all the makings of a storm system and, as it rotates faster and faster, an eye forms at its centre. The eye — very calm and and with very low air pressure — as you may know, is one of the key characteristics of a tropical cyclone.
So, when does a storm become a cyclone: The textbook definition is that when winds in this rotating mass of clouds reach speeds of 63 kmph, the storm is called a ‘tropical storm’. At 119 kmph, the storm is classified as a ‘tropical cyclone’.
But if you’re wondering where the classifications of extreme and catastrophic come from you have to consult the Saffir-Simpson Hurricane Scale, arguably the most common yardstick for describing cyclones. Cyclones are divided into categories depending on the strength of the winds it unleashes. The Saffir-Simpson scale is a five-tier table that maps severity from 1-5. The idea behind thus classifying cyclones is to tie it up with the potential impact it has and the damage it can cause through flooding after making landfall.
How do cyclones move and how do they make landfall?
The will-it-won’t-it uncertainty about where exactly a cyclone is to make landfall is largely due to the fact that although they are brought into being by a vacuum, cyclones do not exactly move in one. That is, cyclones are very much influenced by their interaction with the land and surrounding air masses, which is what is mainly responsible for changes in their route and intensity as they move from sea to land.
Experts say that even “small changes in the initial environment surrounding a cyclone could make large differences in its eventual path, making it harder to predict its movement because it is difficult to work out exactly what the conditions are at sea".
Smaller cyclones can be controlled by lower to mid-levels of the atmosphere, but well-developed cyclones are influenced by large-scale air masses, which is what can cause them to take an erratic path. Experts add that since “cyclones are rotating storms on a rotating planet", they tend to move towards the poles and to the west.
Is there a way to know how much damage a cyclone can cause?
They might appear to roar through the coast, but tropical cyclones normally weaken as they make landfall since they are no longer being fed by the rising air from warm ocean waters. But the rain and the gusting winds they bring in their wake can cause severe damage to life and property.
According to a paper put out by the New Orleans-based Tulane University, “the amount of damage that occurs when a cyclone approaches a coast depends on the angle of approach".
A cyclone that moves along, or parallel, to the coast would cause the bulk of the damage along the coastline closest to the storm, “with bands of lesser damage extending inland". A cyclone that makes a perpendicular approach to the coast “would produce extreme damage all along the right-hand side of its track, with bands of decreasing damage occurring both to the left and right of the track". Also, as it approaches, coastal areas to its right would receive the heaviest thunderstorm activity.
Who comes up with names of cyclones?
Cyclones, hurricanes and typhoons are all, of course, the same type of tropical storm that are known differently in different parts of the world. While hurricanes are tropical storms that form over the North Atlantic Ocean and Northeast Pacific, the same phenomenon in the South Pacific and Indian Oceans is called a cyclone and a typhoon when it is the Northwest Pacific Ocean.
According to the World Meteorological Organisation (WMO), names are necessary for tropical cyclones because there can be more than one such system operating in a particular zone. The names, it adds, are set according to the rules at regional level. In the Atlantic and in the Southern hemisphere (Indian ocean and South Pacific), tropical cyclones names are given in alphabetical order, and women and men’s names are alternated.
For example, Yaas, the name given by means a tree that has a good fragrance and comes close to the English jasmine.
The WMO maintains the rotating lists of names, which are appropriate for each tropical cyclone basin. If a cyclone is particularly deadly or costly, then its name is retired and replaced by another. Generally, the name list is proposed by the National Meteorological and Hydrological Services (NMHS) of the WMO members of a specific region and approved by the respective tropical cyclone regional bodies at their annual or biennial sessions.
Worldwide there are six regional specialised meteorological centres (RSMCs) mandated for issuing advisories and naming tropical cyclones. The Indian Meteorological Department is one of the six RSMCs mandated to provide tropical cyclone advisories to 13 member countries, including Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE, and Yemen.
RSMC, New Delhi, is also mandated to name the tropical cyclones developing over the north Indian Ocean, including the Bay of Bengal and the Arabian Sea.